The invention is in the field of methods for electrochemical generation of gases. More particularly this invention relates to the generation of nitrogen gas from triazole and tetrazole derivatives, and mechanical transducers actuated by the nitrogen gas so produced, particularly in the field of fluid dispensers.
The controlled electrolytic generation of gases is useful to convert chemical to mechanical energy in a variety of applications. For example, a variety of lubricant or fluid delivery systems driven by the electrolytic generation of a gas are known. For example, U.S. Pat. No. 4,023,648 to Orlitzky et al. (1977) shows a lubricant applicator driven by gas generated in an electrochemical cell and provides a method for the electrochemical generation of hydrogen gas.
Fluid dispensers driven by electrochemically generated gases, and other electrochemical transducers may often be used in circumstances which give rise to special operational requirements. Typically, components of any electrolytic cell used in such an application must be stable over time and over a range of temperatures. In such devices, it may be undesirable to have highly reactive gases generated, such as hydrogen or oxygen. Once the circuits are closed to initiate electrolytic gas generation, it is generally desirable to have relatively fast electrode reactions with low overpotential (i.e. a small difference between the electrode potential under electrolysis conditions and the thermodynamic value of the electrode potential in the absence of electrolysis), small concentration polarisation of solutes across the cell (i.e. rapid diffusion of reactants to the electrode surfaces), and small separator resistance effects (i.e. little resistance caused by solid separators within the cell. It is also desirable to produce gases from a small amount of material, i.e. to have efficient gas generation and high stoichiometric coefficients for gaseous reaction products.
The electrochemical generation of a gas can be represented by equation (1):
aR+/xe2x88x92n exe2x88x92xe2x86x92bG+cP 
where R, G and P represent the reactant, the gas product, and the non-gas product respectively; and a, b, c, and n are the stoichiometric coefficients. When utilizing an electrical circuit to drive the current through the electrochemical cell it is desirable to produce gas in an efficient manner from a viewpoint of electric charge consumption. Such efficiency requires a high gas product stoichiometric coefficient associated with a low electron stoichiometric coefficient. A stoichiometric efficiency of gas generation (E) in moles per Faraday may be defined in equation (1) as:
E=b/n mol/F 
Hydrogen and oxygen gases are used in a variety of known electrochemical gas generators. For example the anodic oxidation and cathodic reduction of water respectively generate oxygen and hydrogen by the reactions 1 and 2:
The anodic oxidation of water has a low stoichiometry efficiency for gas production (0.25 mole of oxygen gas per Faraday). A low stoichiometry efficiency may be undesirable because the quantities of reactant and current needed to produced the desired amount of gas may require a large volume of the unit and a high capacity energy source. Another disadvantage of oxygen is that it may pose safety problems when utilized for dispensing combustible fluids such as grease.
The cathodic reduction of water has a better stoichiometric efficiency for gas production (0.50 mole of hydrogen gas per Faraday) but the production of hydrogen gas is hazardous due to its explosive reactivity with oxygen upon ignition. Another disadvantage of hydrogen is that it diffuses relatively rapidly through a variety of polymeric barriers that might otherwise be used to contain the electrolytically generated gas in a mechanical transducer, such as a fluid dispenser.
Nitrogen is a relatively inert gas that may usefully be produced by electrolytic reactions to provide controlled amounts of gas. U.S. Pat. No. 5,567,287 issued to Joshi et al. (1996) discloses a solid state electrochemical nitrogen gas generator for fluid dispensing applications wherein nitrogen is produced by electro-oxidation of alkali metal nitrides or azides. The azide half-cell reaction in that system produces non-reactive nitrogen with a stochiometry efficiency of 1.5 moles of nitrogen gas per Faraday (reaction 3).
2N3xe2x88x92xe2x96xa13N2+2exe2x88x92xe2x80x83xe2x80x83reaction 3 
Based on reaction 3, a fluid dispenser operating at 0.25 mA has the potential to generate about 0.33 ml STP of gas per hour for up to 4000 hours from a battery with capacity of 1 A.h. With sodium azide as the anode reactant, 1 liter STP of nitrogen gas could be generated from about 2 grams of NaN3.
The azide half-cell reaction in such a system may however be slow, in part because of the high overpotential required for the electro-oxidation of azide. To overcome the problem of the sluggish kinetics of the azide half-cell, additives such as thiocyanate may be used to catalyse iodine mediated formation of nitrogen from azides. However, such systems suffer from the disadvantages that azides are toxic and the thiocyanate salt catalysts are also toxic. The presence of toxic compounds may make it difficult to dispose of a device which generates nitrogen gas from azides.
U.S. Pat. No. 6,299,743 to Oloman et al. (2001) discloses the electrochemical generation of nitrogen gas from organic nitrogen compounds, such as hydrazides (RCONHNH2), the corresponding organic hydrazino-carboxylates (RCO2NHNH2) and amino-guanidine salts (e.g. aminoguanide bicarbonate H2NNHC(NH)NH2.H2CO3). For example, the electro-oxidation of methyl hydrazinocarboxylate generates nitrogen gas with a stoichiometric efficiency of 0.5 moles per Faraday according to the putative reaction 4:
CH3CO2NHNH2xe2x88x92 greater than CH3CO2H+N2+2H++2exe2x88x92xe2x80x83xe2x80x83reaction 4 
Based on reaction 4 an electrical source with a current of at least 0.75 mA would be required to generate 0.33 ml STP/hour of nitrogen and a mass of 4 gram of methyl hydrazino-carboxylate would be needed to produce 1 liter STP of the gas.
Compounds having a high nitrogen content such as triazoles and tetrazoles have been investigated as non-azide nitrogen gas generant components in pyrotechnic compositions that may be useful as propellants or for inflating aircraft or automobile safety crash bags. Clearly, the explosive release of gases is not desirable in controlled electrolytic gas generators.
In one aspect, the invention provides electrolytes for the electrochemical generation of nitrogen gas by anodic oxidation of azole derivatives having a high nitrogen content. A high nitrogen content azole compound or derivative refers, in some embodiments, to a five-membered N-heterocycle containing two double bonds and having at least three nitrogen ring atoms. In alternative embodiments, the high nitrogen content azole derivatives of the invention may include triazoles, aminotriazoles, tetrazoles, aminotetrazoles and their salts. A variety of triazoles and tetrazoles may be used and empirically tested for performance in alternative embodiments.
The triazoles may include, for example, the 1H-and 2H-1,2,3-triazole tautomers, the 1H- and 4H-1,2,4-triazole tautomers, and their mono-, di- or trisubstituted derivatives. The mono-, di-, and trisubstituted derivatives may include, for example, suitable alkyl, alkenyl, alkynyl, arylalkyl or aryl groups. The alkyl, alkenyl, and alkynyl groups may be linear or branched, substituted or unsubstituted. In some embodiments, the mono-, di-, and trisubstitued derivatives may include lower alkyl, lower aryl and arylalkyl groups. Lower alkyl and lower arylalkyl groups denote alkyl groups and alkyl moiety in arylalkyl groups having up to and including 4 carbon atoms. Lower alkyls may, for example, include, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secondary butyl or tertiary butyl. Lower arylalkyl may include, for example, benzyl. Aryl groups, for example, may include phenyl and phenyl substituted by up to and including 3 lower alkyl groups as defined above.
The aminotriazoles may include, for example, 1-amino-1H-1,2,3-triazole, 2-amino-2H-1,2,3-triazole, 4-amino-1H or 2H-1,2,3-triazole, 5-amino-IH-1,2,3-triazole, 3-amino-1H or 4H-1,2,4-triazole, 4-amino-4H-1,2,4-triazole and 5-amino-1H-1,2,4-triazole and their mono-, di-, tri and tetrasubstituted derivatives. Mono-, di-, tri- and tetrasubstituted aminotriazoles may include, for example, alkyl, alkenyl, alkynyls, aryl or arylalkyl groups. In some embodiments the mono-, di- tri- and tetrasubstituted aminotriazoles may include lower alkyl, lower arylalkyl and aryl groups, wherein the lower alkyl and lower arylalkyl and aryl groups are defined as previously. Monosubstituted aminotriazoles may include species substituted at the triazole ring and compounds substituted at the amino group. Disubstituted aminotriazoles may include compounds substituted at the triazole ring and amino group, compounds disubstituted at the triazole ring and compounds disubstituted at the amino group. Trisubstituted aminotriazoles may include species disubstituted at the triazole ring and monosubstituted at the amino group and compounds monosubstituted at the triazole ring and disubstituted at the amino group.
The tetrazoles may include, for example, the 1H- and 2H- tautomers and their mono- or disubstituted derivatives. Monosubstituted derivatives may include species substituted at the 1-H or 2-H position on the tetrazole ring or 1H- or 2H-tetrazoles substituted at position 5, i.e. the carbon ring atom. Disubstituted derivatives may include 1,5- or 2,5-disubstituted compounds. Monosubstituted and disubstituted derivatives may include alkyl, alkenyl, alkynyl or arylalkyl or aryl groups. The alkyl, alkenyl and alkynyl groups may be branched or unbranched, substituted or unsubstituted. In some embodiments, the mono- and disubstituted derivatives may include lower alkyl, lower aryl and arylalkyl groups. Lower alkyl and lower arylalkyl groups denote alkyl groups and alkyl moiety in arylalkyl groups having up to and including 4, carbon atoms. Lower alkyls may include, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secondary butyl or tertiary butyl. Lower arylalkyl, may include, for example, benzyl. Aryl groups, for example, may include phenyl and phenyl substituted by up to and including 3 lower alkyl groups as defined above.
The aminotetrazoles may include, for example, 1-amino-1H- tetrazole, 2-amino-2H-tetrazole, 5-amino-1H- tetrazole and 5-amino-2H-tetrazole and their monosubstituted, disubstituted and trisubstituted derivatives. Mono-, di- and trisubstituted aminotetrazoles may include, for example, alkyl, alkenyl, alkynyl, arylalkyl or aryl groups. The alkyl, alkenyl and alkynyl groups may be branched or unbranched, substituted or unsubstituted. In some embodiments, the mono-, di- and trisubstituted aminotetrazoles may include lower alkyl, lower aryl and arylalkyl groups. Lower alkyl and lower arylalkyl groups denote alkyl groups and alkyl moiety in arylalkyl groups having up to and including 4 carbon atoms. Lower alkyls may include, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, secondary butyl or tertiary butyl. Lower arylalkyl, may include, for example, benzyl. Aryl groups, for example, may include phenyl and phenyl substituted by up to and including 3 lower alkyl groups as defined above. Monosubstituted aminotetrazoles may include species substituted at the tetrazole ring or compounds substituted at the amino group. The disubstituted aminotetrazoles may include compounds substituted at the tetrazole ring and amino group and compounds disubstituted at the amino group.
Salts of triazoles, aminotriazoles, tetrazoles and aminotetrazoles include inorganic salts, for example, ammonium, aluminium; alkali metal salts, for example, lithium, sodium or potassium; alkaline earth metal salts, for example, calcium or magnesium; and organic salts, for example, quaternary ammonium salts.
Some such compounds may not work in all embodiments, as determined by routine functional testing. The utility of such compounds may, for example, be routinely assayed in accordance with the guidance provided herein, including the Examples set out herein in which alternative nitrogen compounds may be substituted for routine test purposes.
In another aspect, the electrolyte may function as or further comprise a cathode depolariser reactant to suppress cathodic hydrogen generation. The cathode depolarizer may include, for example, isonicotinic acid and soluble salts thereof (alkali or ammonium for example), nitro-ethanol, nitromethane, nitroguanidine, nitrate salts and chlorate salts.
The invention also provides electrolytic cells incorporating an electrolyte comprising an active nitrogen compound selected from the group consisting of triazoles, aminotriazoles, tetrazoles, and aminotetrazoles wherein the active nitrogen compound is an anode reactant. In some embodiments, the electrolyte may also function as or comprise a cathode depolariser. The cathode depolariser may include, for example, isonicotinic acid and soluble salts thereof such as alkali or ammonium salts for example, nitro-ethanol, nitromethane, nitroguanidine, nitrate salts and chlorate salts.
The electrolytic cells may be associated with a fluid dispenser actuated by nitrogen gas produced at the anode by electrolysis of the active nitrogen compounds of the invention.